Under normal conditions the pneumatic tires on a vehicle in motion undergo a certain degree of deflection from their unloaded round condition. However, undue deflection, such as that caused by a low pressure condition or vehicular overload, places undesirable stresses on the tire, causes the tire tread to wear faster and is otherwise a hazard to the safe operation of the vehicle. Such undue deflection may go undetected for hours in situations involving long trips or the like.
A number of devices have been developed to detect low pressure conditions, but all have their drawbacks. Numerous devices utilize radio circuits to provide a low pressure warning. These devices usually require power sources at each wheel to operate a transmitter with the receiver being in the cab of the vehicle. When a low pressure condition is detected, usually by some type of pressure switch or other device communicating with the tire pressure, the transmitter is activated to warn the operator. Such operation is highly susceptible to false alarms from other transmitters or system failure due to the fact that electromagnetic waves can quite often be blocked by the metallic portions of the vehicle body, particularly in the situation involving a tractor-trailer.
Another type of device senses low pressure conditions by counting tire revolutions and comparing such revolutions with the revolutions of a properly inflated tire to determine if a tire has a smaller diameter than the properly inflated tire which would be indicative of a low pressure condition. These devices require a great deal of sophisticated electronic equipment in order to have any degree of accuracy and reliability and are therefore economically unfeasible.
A more feasible and successful device is that shown in U.S. Pat. No. 4,086,564. There a pressure switch continually monitored the tire pressure. A light source at each wheel directed light off of a reflective surface associated with the pressure switch. Under normal pressure conditions the reflected light would be received by a photo transistor once each revolution of the tire. Under abnormal conditions the pressure switch would change the angular orientation of the reflective surface or otherwise block the light so that a sophisticated electronic circuit would alert the operator of the vehicle. Typical of the pressure switch arrangement as described is shown in U.S. Pat. No. 4,004,272.
While successfully operable, the arrangement of these patents required a light source, reflective surface and pressure switch at each wheel which engendered maintenance problems in replacing dead light sources and periodically cleaning at least the reflective surface. In addition, like other prior art, the sophisticated electronic circuitry rendered the system economically unfeasible.
In U.S. Pat. No. 4,004,270 a simpler electronic circuit is disclosed which would tend to assuage some of the above mentioned problems. There it was suggested, for example, that a pair of brushes contacting once each revolution of the tire to close a circuit when a pressure switch indicated a low pressure condition could replace the optical system just described. This system still, however, required a pressure switch at each wheel and in addition was suseptive to further maintenance problems as the brush contacts would periodically have to be replaced.
Finally, in both the devices of U.S. Pat. Nos. 4,004,270 and 4,086,564, even when operating perfectly, only a low pressure condition could be sensed. Undue tire deflection due to a vehicular overload could go undetected.